The present invention relates to a droplet depositing apparatus and to a method for depositing droplets, notably to a modified ink jet printing apparatus and ink jet printing method.
In a typical continuous jet ink jet printing apparatus, an ink composition is ejected through a nozzle to form a jet of ink which is broken up into substantially uniformly sized droplets by applying a suitable frequency vibration to the ink. The vibration is typically generated by causing a piezo-electric crystal to vibrate by applying a voltage thereto. The droplets are charged by passing them past a charge electrode which imparts a desired charge to each droplet. The charged droplets are deflected by passing them through an electric field, usually generated by a pair of electrically charged deflector plates. The deflection causes the droplets to follow a flight path which either carries the droplets into a catching arrangement so that they do not strike the substrate to be printed and/or causes the droplets to be displaced to a desired extent to form a symbol on the substrate which can be moving relative to the droplets or stationary with the droplets being deflected relative to the substrate.
Such an apparatus is denoted herein as "a droplet depositing apparatus of the kind described".
The extent of deflection is controlled by varying the charge given to each droplet and/or by varying the strength of the deflecting field. However, whichever method is used to control the deflection of the droplet, it is necessary to ensure that each droplet has an essentially consistent mass and composition. If either of these factors varies, the charge per unit mass induced in the droplets will vary. This will affect the deflection and hence the flight path of the droplets and this in turn will affect the deposition pattern of the droplets on the substrate.
During operation of the printer, ink which is caught and not allowed to strike the substrate is recycled through the printing system. With time, the ink loses solvent and other volatile components and its specific gravity and composition change. In order to reduce the effects of these losses from the composition, it has been proposed to monitor the weight of the ink held in the printing system. From a knowledge of the starting weight of ink in the system and the number of characters printed, it is possible to determine the weight of ink which should remain in the system at any given time. The shortfall in the actual amount present represents approximately the weight of solvent lost from the system. The requisite amount of solvent can then be added to the ink reservoir to make good the losses and thus return the ink to the intial composition. Alternatively, the operator merely assesses the number of characters printed and based on an estimate of the solvent losses adds an aliquot of solvent to the ink reservoir at intervals during the operation of the printer.
However, such methods of operation are haphazard and often require that the system be shut down and all the ink drained into the weighing vessel. This is inconvenient and interrupts printing operations. Furthermore, in practice such a system can only be carried out at lengthy intervals in the printing operation, with the result that the compositon of the ink can vary by comparatively large amounts before the need to rectify the position can be verified.